Testing of the central vision (“in the line of sight”) is necessary for diagnosing and following up a wide spectrum of disorders which either destroy, disturb, or disconnect the neural units that make up the visual system. Examples of such disorders include inflammations and degenerations in the macula of the retina, inflammations in the visual nerve, such as multiple sclerosis, and tumour formations pressing against, for instance, the optic nerve or optic chiasm, such as pituitary tumours. In the following the need for improved diagnostics will be developed in more detail using a common disorder as an example.
There is a rapidly increasing number of individuals all over the world with double-sided visual loss due to degeneration of the macula of the eye, referred to as Age Related Macular Degeneration, ARMD. ARMD is the leading cause of blindness in elderly people. Many people know or will soon know somebody suffering from ARMD. ARMD causes an accelerating degradation of visual acuity. ARMD causes in the final stage a serious handicap in everyday life. For instance, the possibilities of reading, watching TV and driving a car are eliminated.
Great efforts are made throughout the world to understand the development of ARMD and to an effective treatment. Promising forms of treatment have just appeared and the search for new drugs is very intense. For optimal results, the treatment must be given at an early stage. It is thus necessary to detect ARMD at an early stage. An early diagnosis and meaningful measurement are key issues in research on and treatment of ARMD.
The most commonly applied test is the classic visual acuity chart that was created around 1850. A patient in the final phase of ARMD has to hold the chart at a reading distance to be able to see the largest letters. The visual acuity chart is an excellent tool when testing spectacles but suffers from great drawbacks when diagnosing and measuring nerve and retinal disorders, including ARMD.
It is established that normal visual acuity can be upheld even with 50% loss of neural units. The problem with visual acuity charts and other similar tests is that the letters contain an excess of information. The test letters can be recognized even if some parts of the letters are invisible. In addition, natural unconscious eye movements assist in filling in the missing parts of the letters, thus making diagnosis of ARMD still more difficult.
The dividing line between central vision (in the line of sight) and peripheral vision (field of vision) is traditionally placed at about 3°, which is at the outer edge of the macula of the retina. Of course, the limit is artificial: vision is “seamless” over the entire field of vision.
Completely different types of tests are used for central and peripheral vision. This is due to the fact that it is difficult to determine what is seen in peripheral vision. For example, it is difficult to read letters that are presented outside about 3° C. The predominant peripheral technique, perimetri, is based on light spots projected in predetermined positions on the inside of a grey hemisphere. The test aims at finding the faintest contrast that can be discovered at each test point (typically about 50 test points). Contrast sensitivity is bracketed with repeated presentations in one and the same position. The predominant test in central vision is visual acuity. As mentioned above, in most cases letter charts are used and here, too, a function level is bracketed by using different letter sizes.
All conventional peripheral and central tests thus use forking techniques in order to specify a function level. The observed function levels can then be compared with normal data. Normal data are widely spread, as are most human characteristics.